Chiral nematic, also known as cholesteric, liquid crystalline materials are useful in a variety of applications including various liquid crystal display components, reflective films, optical filters, polarizers, paints, and inks, among others. Methods for preparing such materials are well established. See, e.g., G. Gottarelli and G. Spada, Mol. Cryst. Liq. Crys., 123, 377 (1985); G. Spada and G. Proni, Enantiomer, 3, 301 (1998); and U.S. patent appliction Ser. No. 10/651,692.
While early uses of chiral nematic compositions relied upon mixtures composed mostly of chiral components, more recently, such materials are composed of nematic liquid crystal mixtures combined with small amounts of chiral dopants. In such new compositions, the properties of the nematic host material, for example viscosity, birefringence, electrical anisotropy, and magnetic anisotropy among others, are tailored to the desired usage by altering the chemical composition of the nematic mixture, and then a chiral dopant is incorporated to induce helical twisting, so as to provide the desire chiral nematic pitch. It is apparent that the properties of this chiral nematic composition are therefore a combination of the properties of the nematic host plus those of the dopant. It is further well understood that by reducing the amount of dopant, the properties of the host nematic liquid crystal formulation might be better preserved. Certainly, reducing the concentration of a specific dopant also reduces the pitch of the resulting chiral nematic formulation.
Many uses of chiral nematic compositions require the formulation to reflect or transmit visible light, thus requiring compositions with substantial helical twist, i.e. short helical pitch (“p”). These considerations indicate that dopants that induce large amounts of nematic helical twist per unit concentration are prized. The figure of merit for such materials is its Helical Twisting Power (“HTP” or β).
A dopant material's HTP (β) is defined, in a specified nematic host at a particular temperature, by Eq (1):β=(pcr)−1  (1)wherein the “p” is the measured helical pitch of the doped nematic (em); “c” is the measure of the dopant concentration (usually in terms of mole fraction, weight fraction, or weight percent on a unitless scale, wherein mole fraction and weight fraction is on a scale of 0 to 1); and “r” is the enantiomeric excess of the dopant (on a unitless scale of 0 to 1). Enantiomeric excess (r), defined as the absolute value of the difference in mole fraction (F) of the two enantiomers in a sample, equals |F(+)-F(−)Å. Thus, for a racemic mixture, r equals |0.5-0.5|, which equals 0; for an enantiomerically pure material r equals |1.0-0|, which equals 1; and for a 75% pure mixture the r equals |0.75-0.25|, which equals 0.5. The larger the HTP, the lower the concentration of dopant needed to provide a specific pitch, and thereby yield a particular reflectance or transmission.
The pitch of a chiral nematic formulation can be measured using a variety of optical techniques. For example, see Z. Dogic and S. Fraden, Langmuir, 16, 7820 (2000). The dopant concentration is as formulated and the enantiomeric excess can be measured via chiral high-performance liquid chromatography (HPLC) or nuclear magnetic resonance (NMR) spectroscopy. Typically, for useful enantiomerically pure dopants, their HTP's range from one to several hundred (μm−1). Dopants with twisting power greater than 100 (based on dopant mole fraction) are often described as “high twist” dopants. The discovery of new dopants, particularly high twist dopants, is important to broadening the utility of chiral nematic formulations.
Not only can chiral nematic liquid crystals be formulated to reflect various wavelengths of incident electromagnetic radiation, but it is well understood that that reflected light is circularly polarized, depending upon the sense of chirality of the helical pitch. Thus, a chiral nematic displaying a right-handed helical mesostructure will reflect right-handed incident light. For many applications, it is useful to be able to reflect both right-handed and left-handed senses of circularly polarized light, for example, in a vertically layered structure. It is further well known that enantiomers of a chiral dopant structure induce the opposite polarity of helical rotation and, therefore, afford oppositely polarized light reflections. For this reason, the preparation of enantiomeric pairs of dopants for use in separate light modulating layers can be particularly useful.
There are three general sources for obtaining substantially enantiomerically pure organic compounds for use as dopants or, more likely, as synthetic precursors for dopants: (1) compounds available from natural sources; (2) the preparative separation of racemic mixtures of enantiomers; or (3) chiral synthetic methods that directly afford desired enantiomers. Most commonly, only the latter two methods provide access to both enantiomers of a potential dopant. Natural sources generally provide only one of any enantiomeric pair, reflecting the fundamental chirality of life. Thus, using natural sources for dopants or their precursors can lead to limitations in dopant utility. The discovery of new dopants available from non-natural sources would therefore be especially useful.
In practical applications, several dopants may be incorporated into nematic hosts to provide chiral nematic liquid crystal formulations. This may be done due to chemical incompatibility of the dopants with the host material, to allow for temperature sensitivity compensation or for other reasons. When combining dopants within one chemical class or of various structural classes, the handedness of the helical twist must be taken into account. Thus, the effects of dopants that induce the same handedness of helical twisting are additive. However, if the helical handedness of two particular dopants are antithetical, the effects of these dopant twists will cancel. This is readily apparent considering racemic mixtures of chiral dopants: each enantiomer of the mixture could have a large HTP, however the theoretical twist that one enantiomeric dopant might induce is exactly negated by the contrary helicity of its enantiomer's effect. A liquid crystal formulation, doped with a combination of two or more dopants with varied handednesses of helical twist, will display a twist related to a linear combination of the concentration, HTP, and handedness of each dopant.
Further, it is could be advantageous to control or alter the HTP of a chiral dopant or dopant mixture after formulation of the chiral nematic liquid crystal mixture or during fabrication of a liquid crystal containing device or perhaps even after device manufacture. Previous workers have discovered photochemical methods for such processing. For an early example, see C. Mioskowski, J. Bourguignon, and S. Candau, Chem. Phys. Letters, 38, 456 (1976). In the manufacture of various liquid crystal display components, it can be useful to alter the wavelength of reflected light after the liquid crystal mixture has been incorporated into the device, for example, in a liquid crystal color filter array fabrication. One might envisage coating a blue reflective (yellow transmissive) chiral nematic liquid crystal, then in a subsequent step alter the pitch of the mesophase by inducing structural modifications in the chiral dopant. If such modifications led to reduction in mesophase pitch, shorter wavelength colors could be formed, thus perhaps providing a red reflective (cyan transmissive) coating. Methods for the in situ altering of the pitch of chiral nematic liquid crystal's are needed.
A known approach toward this end is provided by dopant photochemistry. Irradiation of certain chemical structures can afford isomerization or fragmentation reactions. Such reactions have been used to design photoactive dopants. A search of the chemical literature indicates that several varieties of phototunable chiral dopants have been identified. Usually these involve the isomerization of a double bond from a trans-configuration to a cis-configuration (E to Z) or the reverse, e.g. P. Van de Witte, J. Galan, and J. Lub, Liquid Crystals, 24, 819 (1998); R. van Delden, M. van Gelder, N. Huck, and B. Feringa, Adv. Funct. Mater., 13, 319 (2003) and references therein. Early work was accomplished with thermally reversible azobenzene compounds. Work that is more useful has been accomplished using olefins. Epimerization of 1,1′-binapth-2-ol dopants has also been exploited for such uses, i.e. U.S. Pat. No. 5,668,614; S. Campbell, Y. Lin, U. Miller, and L-C. Chien, Chem. Mater., 10, 1652 (1998) and references therein. A particularly interesting series of disclosures by workers at Fuji Photo Film Company have been published, i.e., U.S. Pat. Appl. 2004 019,228 A1; U.S. Pat. Appl. 2003 122,105 A1; U.S. Pat. Appl. 2003 137,632 A1; U.S. Pat. Appl. 2003 111,639 A1; U.S. Pat. No. 6,589,445 B2; U.S. Pat. No. 6,645,397 B2; Jpn. Kokai Tokkyo Koho JP 2003 306,491; Jpn. Kokai Tokkyo Koho JP 2002 179681; these workers used various bis-cinnamate esters of isosorbide, for example,
Ex-1, Ex-2, Ex-3, as the phototunable chiral dopants. The general isosorbide class of chiral dopant has been extensively explored and patented by Merck (Darmstadt, Germany) i.e. U.S. Pat. No. 6,217,792 B1 and related disclosures. As is evident from the above considerations, the effectiveness of molecules as chiral dopants or, more specifically, as phototunable dopants is not readily predictable. It depends upon the subtle interplay of molecular chirality, chirality transfer from the dopant to the host material, and photochemically induce structural alterations.